1. Field of the Invention
The invention relates to a process for producing cast iron containing globular or nodular graphite.
2. Description of the Prior Art
Methods for the production and the principal advantages of cast iron containing globular graphite have been known for a long time, particularly, since the works of Gagnebin. Cast iron containing globular graphite is generally produced by treating magnesium or a magnesium master-alloy with molten iron which can be obtained by melting in any melting furnace. The magnesium master-alloys used in this case contain, in most cases, iron, nickel, calcium and silicon as the alloy partners. The amount of magnesium or magnesium master-alloy to be added is substantially influenced by the considerable amount of magnesium oxidation which takes place. A very significant oxidation occurs because the temperature of the molten base iron is considerably higher than the relatively low boiling point of the magnesium or, respectively, the vapor pressure of the magnesium in the master-alloy exceeds the normal atmospheric pressure of the molten iron.
Considerable effort has been made to counteract this oxidation. For example, pressurized ladles have been developed, i.e., pouring ladles which can withstand an internal working pressure of more than 20 atmospheres and which can be tightly closed. In most cases, these ladles additionally have a special lining depending on the nature of the melt. Furthermore, special immersion containers have been developed for introducing the magnesium or magnesium master-alloy into the base melt.
Another arrangement used in an attempt to reduce the oxidation of the magnesium consists in the use of special pouring ladles which have a feed opening at the bottom of the ladle through which the molten magnesium is pressed into the base melt. Lance injectors were also developed whereby fine-grained magnesium was blown into the melt by means of an inert carrier gas.
The basic problem with all of these solutions is that very elaborate apparatuses are required. Thus, it is not a satisfactory solution of this problem to use apparatuses or arrangements which are complicated and partly trouble-prone or costly. Also, unsatisfactory results were obtained when it was attempted to use coal impregnated with magnesium or to use magnesium alloys of high specific weight and to cover the alloys after they were placed in the empty pouring ladle by pouring molten iron over them or by introducing the alloys to the melt by means of the dipping process. The magnesium or the magnesium master-alloy, after being placed in the empty pouring ladle, was covered by sandwiching between a variety of material, such as, coke, sheet metal scraps, calcium carbide, ferrosilicon, etc., and, subsequently, the pouring ladle was filled with the molten base iron. However, this procedure did not reduce the amount of oxidation sustained. Those same problems apply analogously to processes using covering slags and slag electrolysis.